Many engineering unit operations and processes operate at high pressures, e.g., sometimes exceeding 1000 psig. However, at some point in the process there is a need to decrease the high pressure because of downstream process design or equipment limitations, and/or any combination of these factors. To decrease the pressure from a higher level to a lower level at which downstream equipment and/or unit operations operate, industry commonly uses pressure let-down valves. The pressure let-down valves reduce high pressure fluids out of the particular unit operation to the desired pressure level. Subsequently, the process stream, now at the reduced pressure, is conducted to a down-stream equipment of unit operation. Pressure let-down valves find application in such diverse areas as oil and gas wells, coal liquefaction plants, and synthetic fuels (e.g., methanol-to-gasoline) processes.
One example of a prior art pressure let-down valve is that produced by Willis Oil Tool Company and schematically illustrated in FIGS. 1-3. The Willis pressure let-down valve is available in various sizes, ranging from 1 inch to 8 inches, depending on the desired area of application. A typical Willis pressure let-down valve is comprised of a valve body housing containing a variable choke and, in some cases, a fixed choke. The fixed choke (FIG. 1), when used, is located downstream of the variable choke. The variable choke is comprised of a fixed disc assembly and a rotating disc assembly. The fixed disc assembly and the rotating disc assembly each comprise a substantially circular disc with a number of openings. When the valve is fully opened, the openings in the fixed and in the rotating disc assemblies are in direct alignment with each other (FIG. 2A) to allow the maximum flow of fluids (liquids and/or gases) through the valve. When it is desired to throttle back or decrease the flow rate of fluids through the valve, the variable choke assembly is rotated radially a few degrees from the fully opened position, e.g., by a turning fork (FIG. 1), thereby decreasing the common opening available for the flow of fluids (FIG. 2B). When the desired degree of pressure has been achieved in a process vessel, and it is desired to terminate the flow of fluids therefrom, the turning fork is again turned to cause a complete misalignment of the openings in the rotating and in the fixed disc assemblies (FIG. 2C).
The prior art rotating disc assembly typically comprises a stainless steel retaining ring and a wear-resistant insert having openings therein (FIG. 3). The openings in the wear-resistant insert are aligned with the openings of the fixed disc assembly when a fully opened position of the valve is desired. In prior art, the wear-resistant insert of the rotating disc assembly is attached to the retaining ring by a silver solder joint illustrated in FIG. 3. The retaining ring is usually made of stainless steel and the wear-resistant insert is made from any highly wear-resistant material, such as tungsten carbide. Thus, the silver solder joint has been necessary in heretofore-produced rotating disc assemblies because of dissimilar materials of construction used for the retaining ring and for the wear-resistant inserts. However, in practical applications, the silver solder joint has often failed under process conditions in the first one to six days of operation because of a number of operational factors. For example, different coefficients of expansion of the retaining ring and of the wear-resistant insert, the reduced strength of silver solder at process temperature, and the high sheer stresses imposed on the silver solder joint during throttling operation significantly contribute to the early failure of the silver solder joint. The failure of the joint causes loss of operating control and obvious expensive maintenance costs associated with the need to frequently interrupt the operation of the process to replace or repair the rotating disc assembly.
Accordingly, it is a primary object of the present invention to provide an improved means of attaching a wear-resistant insert to the retaining ring in a pressure let-down valve.
It is an additional object of the present invention to eliminate the silver solder joint between the wear-resistant insert and the retaining ring in a pressure let-down valve, thereby eliminating a major maintenance expense.
Additional objects of this invention will become apparent to those skilled in the art from the study of the specification and of the appended claims.